Highly sensitive particle enhanced assay for the quantification of nt-probnp

ABSTRACT

NT-proBNP can be determined in a biological sample using at least one antibody which recognizes an epitope of NT-proBNP in both a glycosylated and non-glycosylated form of NT-proBNP. Said antibody is preferably an isolated polyclonal antibody or a mixture of monoclonal antibodies coated onto a particle, preferably coated onto said particle in a coating ratio of 6-60%, forming a layer or multiple layers of antibodies on said particle. The assay, realized in the form of a nephelometric or turbidimetric assay, can be applied to a wide range of automated clinical analyzers.

TECHNICAL FIELD

This disclosure relates to the field of diagnostic assays, and inparticular to a novel assay for determination of a concentration ofN-terminal pro-hormone BNP (NT-proBNP) in a sample, includingantibodies, immunoparticles, reagents and calibrators for performingsuch assay.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has beensubmitted electronically in XML format and is hereby incorporated byreference in its entirety. Said XML copy, created on Feb. 3, 2023, isnamed “51398-006002_Sequence_Listing_2_3_23.xml” and is 9.3 kilobytes insize.

BACKGROUND

Brain natriuretic peptide, also known as B-type natriuretic peptide,abbreviated BNP, is a hormone secreted by cardiomyocytes in the heartventricles in response to stretching caused by increased ventricularblood volume, cardiac wall stress. NT-proBNP is produced by cleaving theproBNP into BNP and the NT-proBNP. See FIG. 1 .

Measuring BNP and NT-proBNP is useful in the diagnosis and monitoring ofheart failure. BNP and NT-proBNP cannot be compared directly, as BNP isthe biological active metabolite and has a half-life (t_(1/2)) ofapprox. 20 min and does not degrade in the kidneys. It is only stablefor a very short time in vitro.

NT-proBNP is however more stable, it is biologically inactive andfiltered by the kidneys. Its half-life is about 1 to 2 hours and theconcentration in the blood is higher than that of BNP. Compared to BNP,the half-life is highly dependent of kidney function. In vitro,NT-proBNP is more stable than BNP, and can be stored for at least threedays at 2-8° C. Consequently, proBNP and NT-proBNP are used as goldstandard clinical markers of myocardial dysfunction such as cardiachypertrophy and left ventricle heart failure.

The recommended cut-off to exclude heart failure in patients withnon-acute symptoms is 35 ng/L and 125 ng/L for BNP and NT-proBNPrespectively. The recommended cut-off to exclude heart failure inpatients with dyspnea or increasing symptoms is higher, 100 ng/L and 300ng/L for BNP and NT-proBNP respectively. The reference range is howeverwide, in particular for acute heart failure, and in particular for olderpatients, see Table 1:

TABLE 1 Age-stratified cut-offs for the diagnosis of acute heart failureAge Cut-off Sensitivity, % <50 years  <450 pg/mL 97 50-75 years  <900pg/mL 90 >75 years <1800 pg/mL 85 (Januzzi et al., NT-proBNP testing fordiagnosis and short-term prognosis in acute destabilized heart failure:an international pooled analysis of 1256 patients. Eur Heart J 2006; 27:330-37)

NT-proBNP is a peptide chain of 76 amino acids with a mass of 8458 Da inits non-glycosylated form. The amino acid sequence is shown in FIG. 2 .Before 2006, the glycosylation of proBNP was unknown. The first evidenceof proBNP glycosylation was published in 2006 (Schellenberger, U. etal., The precursor to B-type natriuretic peptide is an O-linkedglycoprotein, Arch Biochem Biophys. 2006 Jul. 15; 451(2):160-6. Epub2006 April 19). The presence of glycosylation and the observations ofhigher molecular weight isoforms of BNP have caused much confusion aboutthe relevant forms of NT-ProBNP in the blood stream.

In 2015, it was shown that higher NT-proBNP concentrations were measuredusing the proBNP II assay on an Elecsys® platform (Roche Diagnostics)when the samples were pretreated with a deglycosylation enzyme (Røsjø etal., Influence of glycosylation on diagnostic and prognostic accuracy ofN-terminal pro-B-type natriuretic peptide in acute dyspnea: data fromthe Akershus Cardiac Examination 2 Study, Clin Chem, 2015 August;61(8):1087-97. doi: 10.1373/clinchem.2015.239673. Epub 2015 Jun. 8).

The exact glycosylation sites of endogenous proBNP and NT-proBNP wereonly recently characterized (Halfinger, B. et al., Unravelling theMolecular Complexity of O-Glycosylated Endogenous (N-Terminal)pro-B-Type Natriuretic Peptide Forms in Blood Plasma of Patients withSevere Heart Failure. Clinical Chemistry. 2017, Vol. 63, 1).

Sandwich immunoassays for the detection of NT-proBNP are commerciallyavailable, and disclosed for example in EP 1151304 (Roche DiagnosticsGmbH) which relates to a sandwich assay using two antibodies whichrecognize different epitopes of the native NT-proBNP and at the sametime can bind to said native NT-proBNP, wherein the at least twoantibodies bind in the region of amino acid 10-50 of N-terminal proBNPand the antibodies are obtained by immunizing a suitable organism withrecombinant NT-proBNP.

EP 1625163, EP 1625164 and EP 2256132 (Roche Diagnostics GmbH) defineantibodies specific to epitopes on the proBNP molecule defined by aminoacids no. 38-43, 42-46, 39-66, 40-47, 41-48 and 42-49.

U.S. Pat. Nos. 9,034,591 and 9,145,459 (HyTest Ltd.) both relate to animmunoassay kit for detecting BNP, proBNP, or a fragment thereof, in asample, comprising: a first antibody, or binding fragment thereof,specific for a region of a ring structure of BNP and proBNP; wherein thefirst antibody, or binding fragment thereof, binds to BNP, proBNP, or toa fragment thereof comprising the region thereof so as to form a firstorder immune complex; and a second antibody, or binding fragmentthereof, not recognizing free BNP, free proBNP, or a free fragmentthereof, or free first antibody, or which recognizes them with 10-foldor less affinity than it recognizes the first order immune complex,wherein, the second antibody, or binding fragment thereof, is Ab-BNP2 orAb-BNP4.

EP2084544B1 (HyTest Ltd.) relates to stable standards for BNPimmunoassays, and defines the use of a peptide selected from the groupconsisting of an isolated or recombinant or synthetic proBNP consistingof a specific amino acid sequence or a sequence that differs by five orfewer amino acid substitutions, insertions or deletions as a standard orcalibrator in a method for detecting BNP immunoreactivity in a sample.

EP2251356B1 (Nexus DX Inc.) relates to a polyclonal-monoclonal ELISAassay for detecting N-terminal proBNP using isolated polyclonalantibodies specific for NT-proBNP within a certain range of amino acidresidues, and isolated monoclonal antibodies specific for NT-proBNPwithin another, specific range of amino acid residues.

Based on the above, a need still exists in the field to quicklydetermine the correct or true NT-proBNP concentration by a sensitivemethod.

SUMMARY

The present inventors solve the problem that currently available assaysare not sufficiently sensitive and most likely underestimate the trueNT-proBNP concentrations by providing an improved particle-enhancedimmunoassay for the quantification of NT-proBNP.

NT-proBNP, the N-terminal region of proBNP that consists of 76 aminoacids and six (seven) O-glycosylation sites, has the following aminoacid sequence:

HPLGSPGSAS DLETSGLQEQ RNHLQGKLSE LQVEQTSLEP LQESPRPTGV WKSREVATEGIRGHRKMVLY TLRAPR (SEQ ID NO. 1).

The inventors have identified two novel regions of the NT-proBNP,comprising a part or all of the following amino acid residues:

SAS DLETSGLQEQ RNHLQGKLSE LQV  (SEQ ID NO. 2, Peptide 1, aminoacid residues 8-33 of SEQ ID NO. 1),  and IRGHRKMVLY TLRA (SEQ ID NO. 3, Peptide 2, amino  acid residues 61-74 of SEQ ID NO. 1).

Accordingly, a first aspect of the present disclosure concerns a methodfor determining the concentration of N-terminal pro-brain natriureticpeptide (NT-proBNP) in a sample, said method comprising the followingsteps:

-   -   contacting the sample with at least one antibody which        specifically binds to NT-proBNP within the amino acid sequence        of SEQ ID NO. 2; and/or    -   contacting the sample with at least one antibody which        specifically binds to NT-proBNP within the amino acid sequence        of SEQ ID NO. 3;        wherein said at least one antibody is immobilized to a particle;        and    -   determining a change in reflectance, scattering or transmittance        of the sample wherein said change is indicative of the        concentration of NT-proBNP in the sample.

According to one embodiment of said first aspect, said at least oneantibody specifically binds to NT-proBNP within the amino acid sequenceof SEQ ID NO. 2.

According to another embodiment of said first aspect, said at least oneantibody comprises a polyclonal antibody and/or a mixture of monoclonalantibodies.

As used herein, the term “antibody” refers to polyclonal antibodies, amixture of monoclonal antibodies of any isotype (IgA, IgG, IgD, IgM,IgY) or an antigen-binding fragment thereof, including but not limitedto F(ab), F(ab′), F(ab′)₂, Fv fragments, single chain antibodies such asscFv, chimeric antibodies, humanized antibodies and a Fab expressionlibrary.

According to another embodiment of the above aspect and embodimentsthereof, the polyclonal antibody and/or mixture of monoclonal antibodiesbinds with substantially same affinity to both glycosylated andnon-glycosylated forms of NT-proBNP or fragments thereof. Saidglycosylated and non-glycosylated forms of NT-proBNP or fragmentsthereof can be glycosylated and non-glycosylated recombinant NT-proBNPor fragments thereof, or glycosylated and non-glycosylated nativeNT-proBNP or fragments thereof.

According to another embodiment of the above aspect and embodimentsthereof, said polyclonal antibody and/or the mixture of monoclonalantibodies binds/bind specifically to NT-proBNP with a K_(D) of lessthan 10.0E-09 M, preferably less than 5.0E-09 M, more preferably lessthan 2.0E-09 M. Preferably, the polyclonal antibody and/or the mixtureof monoclonal antibodies binds to recombinant NT-proBNP or fragmentsthereof with a K_(D) of less than 10.0E-09 M, preferably less than5.0E-09 M, more preferably less than 2.0E-09 M.

According to another embodiment of the above aspect and embodimentsthereof, said polyclonal antibody and/or mixture of monoclonalantibodies binds specifically to the peptide of SEQ ID NO. 2 or afragment thereof with a K_(D) of less than 5.0E-09 M, preferably lessthan 2.0E-09 M.

According to an embodiment of the above aspect and embodiments thereof,said polyclonal antibody and/or mixture of monoclonal antibodies bindsspecifically to the peptide of SEQ ID NO. 3 or a fragment thereof with aK_(D) of less than 10.0E-09 M, preferably less than 5.0E-09 M, morepreferably less than 2.0E-09 M.

According to another embodiment of the above aspect and embodimentsthereof, said at least one antibody is a polyclonal antibody or amixture of monoclonal antibodies, preferably a polyclonal antibody,binding to at least one epitope chosen from the epitopes consisting ofthe amino acid sequences of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO.6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9 and SEQ. ID. NO. 10.

In one embodiment, said at least one antibody binds to one epitopehaving a sequence selected from the group consisting of SEQ. ID. NO. 4,SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID.NO. 9 and SEQ. ID. NO. 10. In another embodiment, said at least oneantibody binds to 2, 3, 4, 5 or more epitopes, wherein the epitopes areselected from the group consisting of SEQ. ID. NO. 4, SEQ. ID. NO 5,SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9 and SEQ.ID. NO. 10.

As used herein, the term “sample” refers to a sample of whole blood,blood plasma or blood serum obtained from a subject.

As used herein, the term “subject” refers to a mammal, preferably ahuman.

In a method according the first aspect and any embodiment thereof, theat least one antibody is immobilized to a particle. In one embodiment,the particle has an average (mean) size in the range of 150-300 nm,preferably 150-240 nm, more preferably 160-240 nm, and most preferably180-220 nm, in uncoated form. The average size of the particle may bedetermined using various methods that are known in the art. Oneexemplary method of determining the average size of the particle is byusing by using nanoparticle tracking analysis, for example by using theNanoSight® NS300 instrument (Malvern Panalytical). Nanoparticle trackinganalysis (NTA) is based on the NanoSight®-principle and is a method forvisualizing and analyzing particles in liquids that relates the rate ofBrownian motion to particle size.

As used herein, the term “coating ratio” refers to the weight ratio ofantibodies and particles in the coating process. For example, a coatingratio of e.g. 10%) corresponds to the addition of 1 mg antibodies to acoating reaction with 10 mg latex particles (1:10).

According to one embodiment of the invention, the at least one antibodyis coated onto the particle at a coating ratio (weight ofantibodies:weight of latex particles) of at least 6%, (0.6:10) to 60%(6:10), preferably 6% (0.6:10)-30% (3:10), most preferably 10%(1:10)-20% (2:10).

In one embodiment, the particle is a latex particle, preferably achloromethyl latex particle.

According to another embodiment of the first aspect, and any embodimentsthereof, the change in reflectance, scattering or transmittance isdetermined at a wavelength in the range from 350 to 700 nm, preferablyat about 450-550 nm, such as 546 or 548 nm.

According to another embodiment of the first aspect, and freelycombinable with any embodiments thereof, the method includes a step ofcalibration using a calibrator. As used herein, the term “calibrator”refers to a substance of known concentration, wherein said calibrator ischosen from a peptide with the amino acid sequence of SEQ ID NO. 2, or afragment thereof; or a peptide with the amino acid sequence of SEQ IDNO. 3, or a fragment thereof. In one embodiment, the calibrator has theamino acid sequence of any one of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ.ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9, SEQ. ID. NO.10, SEQ. ID. NO. 11, SEQ. ID. NO. 12, SEQ. ID. NO. 13, SEQ. ID. NO. 14,SEQ. ID. NO. 15, or combinations thereof. In another embodiment, anative or full-length NT-proBNP according to SEQ ID NO. 1 is used ascalibrator. In one embodiment the calibrator is glycosylated. In anotherembodiment, the calibrator is non-glycosylated.

According to preferred embodiment of the first aspect, and freelycombinable with any embodiments thereof, the change in reflectance,scattering or transmittance of the sample is a change in transmittance,and the method is performed as a turbidimetric measurement of theconcentration of NT-proBNP.

According to another embodiment the change in reflectance, scattering ortransmittance of the sample is a change in reflectance or scattering,and the method is performed as a nephelometric measurement of theconcentration of NT-proBNP.

A second aspect of the present disclosure relates to an immunoparticlecomprising a particle as the core coated with at least one antibodychosen from a polyclonal antibody or a mixture of monoclonal antibodieswhich specifically binds to NT-proBNP within the amino acid sequence ofSEQ ID NO. 2; or which specifically binds to NT-proBNP within the aminoacid sequence of SEQ ID NO. 3, preferably wherein said at least oneantibody specifically binds to NT-proBNP within the amino acid sequenceof SEQ ID NO. 2.

According to an embodiment of said second aspect, said polyclonalantibody and/or mixture of monoclonal antibodies binds withsubstantially same affinity to both glycosylated and non-glycosylatedforms of NT-proBNP. Said glycosylated and non-glycosylated forms ofNT-proBNP can be glycosylated and non-glycosylated recombinant NT-proBNPor fragments thereof or glycosylated and non-glycosylated nativeNT-proBNP or fragments thereof.

Preferably said polyclonal antibody and/or the mixture of monoclonalantibodies binds specifically to NT-proBNP and exhibits a K_(D) torecombinant NT-proBNP of less than 10.0E-09 M, such as less than 5.0E-09M, preferably less than 2.0E-09 M.

According to an embodiment, the polyclonal antibody and/or the mixtureof monoclonal antibodies binds/bind specifically to the peptide of SEQID NO. 2 with a K_(D) of less than 5.0E-09 M, preferably less than2.0E-09 M.

According to another embodiment, the polyclonal antibody and/or themixture of monoclonal antibodies binds/bind specifically to the peptideof SEQ ID NO. 3 with a K_(D) of less than 10.0E-09 M, preferably lessthan 5.0E-09 M, more preferably less than 2.0E-09 M.

According to an embodiment of said second aspect, said at least oneantibody is a polyclonal antibody or a mixture of monoclonal antibodies,preferably a polyclonal antibody, binding to at least one epitope chosenfrom the epitopes consisting of the amino acids of SEQ. ID. NO. 4, SEQ.ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO.9, and/or SEQ. ID. NO. 10.

According to an embodiment of said second aspect, freely combinable withall aspects and embodiments herein, said core is a latex core having anaverage size in the range of 150-300 nm, preferably about 150-240 nm,more preferably 160-240 nm, most preferably 180-220 nm in uncoated form,determined by using nanoparticle tracking analysis, for example by usingthe NanoSight® NS300 instrument (Malvern Panalytical). Nanoparticletracking analysis (NTA) is based on the NanoSight®-principle and is amethod for visualizing and analyzing particles in liquids that relatesthe rate of Brownian motion to particle size.

A third aspect of the disclosure relates to a kit for particle enhancedoptical determination of a concentration of NT-proBNP in a sample, saidkit comprising:

-   -   at least one antibody which specifically recognizes and binds to        NT-proBNP within the amino acid sequence of SEQ ID NO. 2 or        within SEQ ID NO. 3, wherein said at least one antibody is        immobilized to a particle;    -   a calibrator, and optionally,    -   instructions for using the kit to determine the concentration of        NT-proBNP in the sample.

According to an embodiment of said third aspect, the at least oneantibody immobilized to a particle forms an immunoparticle according tothe second aspect and any embodiment thereof.

According to an embodiment, the kit includes a mixture ofimmunoparticles wherein said mixture comprises both immunoparticlescoated at least one antibody which binds to NT-proBNP within the aminoacid sequence of SEQ ID NO. 2, and immunoparticles coated with at leastone antibody which binds to NT-proBNP within the amino acid sequence ofSEQ ID NO. 3.

According to an embodiment of said third aspect, the kit comprises astorage buffer in which the immunoparticles are suspended. In apreferred embodiment, said storage buffer comprises a density gradientmedium wherein the density of said storage buffer is adjusted tosubstantially the same specific weight as the specific weight of theimmunoparticles.

According to another embodiment of said third aspect, the kit furthercomprises an assay buffer. In one embodiment, said assay buffercomprises an accelerator, a blocking agent, or a combination of both.

In one preferred embodiment, the accelerator is polyethylene glycol(PEG). In a more preferred embodiment, PEG is present in a concentrationof 0.05-0.5% (wt/vol), preferably 0.05-0.4% (wt/vol).

According to another embodiment, the blocking agent to avoid or reducenon-specific interactions during the immunoreaction, and thereby improvethe accuracy of the immunoassay. In one embodiment, the blocking agentcomprises a protein (e.g. from a bovine species, rabbit, or chicken)preferably at a concentration of 0.001-2%. According to one embodimentthe blocking agent is IgG in a concentration of about 0.1%.

According to another embodiment, said particle enhanced optical methodis chosen from nephelometry and turbidimetry, preferably turbidimetry,and the instructions for using the kit define the steps of performing aturbidimetric or nephelometric analysis.

A fourth aspect of the disclosure relates to a method for identifying asubject with heart failure or having an elevated risk for heart failureby determining the concentration of NT-proBNP in a bodily fluid sampleof said subject, said method comprising:

-   -   providing an immunoparticle according to the second aspect or        any embodiments thereof;    -   reacting the sample with said immunoparticles;    -   detecting a change in reflectance, scattering or transmittance        of the sample, wherein said change in reflectance, scattering or        transmittance of the sample is indicative for the amount of        NT-proBNP;        wherein said change in reflectance, scattering or transmittance        is indicative that the subject has heart failure or an elevated        risk for heart failure.

According to one embodiment, the method further comprises the step ofdetermining the elevated concentration of NT-proBNP above apre-determined concentration. The pre-determined concentration is anage- and sex-dependent concentration of NT-proBNP, and known in the art.

According to one embodiment, heart failure is congestive heart failure,for example congestive heart failure of New York Heart Association(NYHA) Functional Classification classes II, III, or IV (Dolgin M,Association NYH, Fox AC, Gorlin R, Levin R I, New York HeartAssociation. Criteria Committee. Nomenclature and criteria for diagnosisof diseases of the heart and great vessels. 9th ed. Boston, MA:Lippincott Williams and Wilkins; Mar. 1, 1994).

In the above, the sample is a sample chosen from blood plasma and bloodserum and the change in reflectance, scattering or transmittance is achange in transmittance and the method is a turbidimetric method.

In the alternative, the sample is a sample chosen from blood plasma andblood serum and the change in reflectance, scattering or transmittanceis a change in reflectance or scattering and the method is anephelometric method.

A fifth aspect of the disclosure relates to a polyclonal antibody or amixture of monoclonal antibodies, binding specifically to NT-proBNPwithin the amino acid sequence of SEQ ID NO.2, or SEQ ID NO.3. In oneembodiment of the invention, the polyclonal antibody or a mixture ofmonoclonal antibodies exhibits a disassociation constant, K_(D), toNT-proBNP of less than 10.0E-09 M, preferably less than 5.0E-09 M, andmost preferably less than 2.0E-09 M. The disassociation constant can bedetermined using surface plasmon resonance (SPR) technology in a methodoften referred to as the Biacore™ method, well-known to a person skilledin the art.

Preferably said polyclonal antibody or mixture of monoclonal antibodies,is capable of specifically recognizing and binding to both glycosylatedand non-glycosylated forms of recombinant NT-proBNP or fragments thereofwith substantially same affinity.

According to another embodiment of said fifth aspect, said polyclonalantibody or mixture of monoclonal antibodies is preferably capable ofspecifically recognizing an epitope of NT-proBNP within the amino acidsequence of SEQ ID NO. 2, and/or an epitope of NT-proBNP within theamino acid sequence of SEQ ID NO. 3.

Preferably said polyclonal antibody or mixture of monoclonal antibodiesbinds to at least one epitope chosen from the epitopes consisting of theamino acids of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID.NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9 and/or SEQ. ID. NO. 10.

Another aspect of the present disclosure is the use in vitro of apolyclonal antibody or mixture of monoclonal antibodies of the inventionin diagnosis, monitoring, stratifying or predicting mortality rate inpatients with heart failure, in particular congestive heart failure orpatients at risk of developing heart failure, in particular congestiveheart failure.

In the above aspect, said polyclonal antibody preferably binds toNT-proBNP or a fragment thereof within the amino acid sequence of SEQ IDNO. 2 or a fragment thereof, and/or the amino acid sequence of SEQ IDNO. 3 or a fragment thereof. In one embodiment, said polyclonal antibodybinds to a peptide with the amino acid sequence of SEQ ID NO:2 or SEQ IDNO:3. In another embodiment, said polyclonal antibody binds to a peptidewith the amino acid sequence of SEQ ID NO:2 and SEQ ID NO:3.

According to an embodiment of this aspect, said polyclonal antibody ormixture of monoclonal antibodies, preferably polyclonal antibody, bindsto at least one epitope chosen from the epitopes consisting of the aminoacids of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO. 7,SEQ. ID. NO. 8, SEQ. ID. NO. 9 and/or SEQ. ID. NO. 10.

Yet another aspect of the present disclosure is the in vitro use of apeptide of SEQ ID NO. 2; or a peptide of SEQ ID NO. 3, or fragmentsthereof, or a combination thereof, in a method for determining theconcentration of NT-proBNP in a sample, preferably for turbidimetricallydetermining the concentration of NT-proBNP in a sample.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodimentsand features described above, further aspects, embodiments and featureswill become apparent by reference to the following drawings and thedetailed description and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 shows the scheme of proBNP processing (Source: TechNotes|HumanProBNP, BNP and NT-proBNP, HyTest Ltd., January 2019, downloaded07.08.2019).

ProBNP is formed following the translation and cleavage of the signalpeptide of pre-proBNP molecule. It is then glycosylated at severalsites. Two pools of proBNP that are different in the status of T71glycosylation are formed: non-glycosylated at T71 and moleculesglycosylated at this site. Glycosylation suppresses the subsequentprocessing of proBNP. Only proBNP that is not glycosylated at T71 can beeffectively processed into BNP and NT-proBNP. Non-processed proBNP,NT-proBNP and BNP are released into the blood.

FIG. 2 presents the amino acid sequence of NT-proBNP as amino acids27-102 of NCBI Reference Sequence NP_002512, with the two peptidesidentified by the present inventors indicated: amino acid position 8-33(Peptide 1) and 61-74 (Peptide 2).

FIG. 3 shows an example of the Biacore™ affinity analysis for IgGantibody from rabbit antiserum, purified against non-glycosylatedrecombinant NT-proBNP, and tested against a Biacore™ chip withfull-length antigen. Equilibrium dissociation constant K_(D)=1.35E-09 M.

FIG. 4 shows the Biacore™ affinity results for IgY antibody purifiedagainst Peptide 1 and tested against a Biacore™ chip with full-lengthantigen. Equilibrium dissociation constant K_(D)=2.11E-09 M.

FIG. 5 shows the correlation between the Biacore™ affinity constant(K_(D)) and the span response readout achieved with a clinical chemistryanalyser (Mindray® BS-400).

FIG. 6 shows the correlation between the Biacore™ affinity constant(K_(D)) and the sensitivity achieved with a clinical chemistry analyser(Mindray® BS-400).

FIG. 7 shows the size distribution of immunoparticles determined using aNanoSight® NS300 instrument (Malvern Panalytical Ltd., Malvern, UK), theresults confirming that a substantially homogenous lot was achieved. Animmunoparticle lot made with 200 nm latex particles and a 10% coatingratio was used for the measurement.

FIG. 8 shows the difference in particle size (nm, mean) compared touncoated latex particles based on coating ratio. The measurements wereperformed on a NanoSight® NS300 instrument (Malvern Panalytical Ltd.,Malvern, UK).

FIG. 9 schematically illustrates the estimated structure of animmunoparticle carrying an overload of antibodies, based on particlesize measurements. A mean particle size (diameter) increase of about 20nm indicates that one layer of antibodies is present, whereas anincrease of about 40 or about 60 nm indicates two or three layers,respectively.

FIG. 10 shows two calibration curves for 200 nm particles coated withantibodies at a coating ratio of 50% using two different sets ofcalibrators, one fresh and one stored for 2 months at 4° C., confirmingthe stability of the antigen in the buffer.

FIG. 11 illustrates how the calibration curve can be fine-tuned byvarying the ionic strength in the assay buffer.

FIG. 12 shows an example of a calibration curve for lot 46 (200 nmparticles coated with antibodies raised against the peptide of SEQ IDNO. 2 (Peptide 1)), using the same peptide (SEQ ID. 2) as calibratorantigen.

FIG. 13 shows an example of a calibration curve for lot 49 (200 nmparticles coated with antibodies raised towards the peptide of SEQ ID NO3 (Peptide 2)), using native antigen as calibrator antigen.

FIG. 14 is a graph comparing the results (ng/l) of 28 turbidimetricmeasurements of NT-proBNP to the assigned values of the same patientsamples measured using the Roche Elecsys® assay.

FIG. 15 shows the same data as FIG. 14 , here only including sampleswith a measured NT-proBNP concentration lower than 1000 ng/L.

FIG. 16 is a graph showing the correlation between 29 patient samplesmeasured on two different instruments, Mindray® BS-400 and Architectc4000, using the same reagents and calibrators on both instruments.

FIG. 17 is a graph where the response from an assay using a mixture ofimmunoparticles is plotted against the concentration of NT-proBNP.

FIG. 18 is a graph comparing the results (ng/l) of 14 turbidimetricmeasurements using a mixture of immunoparticles against NT-proBNP to theassigned values of the same patient samples measured using the RocheElecsys® assay.

DETAILED DESCRIPTION

Before the present invention is described, it is to be understood thatthe terminology employed herein is used for the purpose of describingparticular embodiments only and is not intended to be limiting, sincethe scope of the present invention will be limited only by the appendedclaims and equivalents thereof.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise.

The term “sample” as in “a sample of whole blood” refers to a sampletaken from a human or animal body, and which sample will not be returnedto said human or animal body. There are standardized methods forobtaining and handling a blood sample taken from a human or animal body,involving the use of needles, syringes, micro cuvettes etc. Thesemethods are well-known to persons skilled in the art. The currently mostpreferred type of sample is lithium heparin treated sample of wholeblood. There are several blood collection tubes containing spray coatedlithium heparin readily available from various commercial supplies, e.g.the BD Vacutainer® available from BD, Oakville, Ontario, Canada.

The term “immunogen” in this disclosure refers to substance used toproduce antibodies and includes conjugated and unconjugated forms.

The term “specifically binds to NT-proBNP within the amino acid sequenceof SEQ ID NO. 2 or SEQ ID NO. 3” is understood to mean that only aminoacids within this amino acid stretch (SEQ ID NO. 2 or SEQ ID NO. 3) ofNT-proBNP are responsible for specifically recognizing and/or binding tothe antibody. In other words, an antibody as described hereinspecifically recognizes and/or binds to an epitope within this aminoacid stretch (SEQ ID NO. 2 or SEQ ID NO. 3) of NT-proBNP.

The term “specifically recognizes an epitope” or “specifically binds anepitope” is understood to mean that only the amino acids of the epitopeare responsible for the binding to the antibody. Accordingly, the term“epitope” denotes the antigenic determinant, namely a specific part orparts of an antigen such as NT-proBNP to which an antibody binds (suchas an epitope comprising part or all amino acids found in the amino acidsequences corresponding to SEQ ID NO: 2 or 3). None of the other aminoacids of an antigen such as NT-pro-BNP are supposed to be involved inthe binding of the antibody. Lack of cross-reactivity of an antibody toother molecules means that the antibody has high epitope specificity.

Further, “binding with substantially the same affinity” is understood tomean that an antibody binds for example to two or more differentpeptides with binding affinities in the same order of magnitude. Theantibody or antibodies as described herein bind to a peptide, e.g. an8-mer peptide of in the context of an epitope mapping assay, or to aprotein, with an affinity of at least about 50%, of at least about 60%,of at least about 70%, of at least about 80%, of at least about 90%, ofat least about 95%, of at least about 97%, of at least about 98%, or ofat least about 99% of the affinity of the same antibody or antibodiesbinding to another peptide or protein.

In another embodiment, an anti-NT-proBNP antibody or antigen-bindingfragment thereof which binds to non-glycosylated NT-proBNP with the sameor substantially the same affinity as to a glycosylated form ofNT-proBNP, e.g. native NT-proBNP.

As used herein, the term “at least one antibody or antibodies” refers topolyclonal antibodies, a mixture of monoclonal antibodies of any isotype(IgA, IgG, IgD, IgM, IgY) or an antigen-binding fragment thereof,including but not limited to F(ab), F(ab′), F(ab′)2, Fv fragments,single chain antibodies such as scFv, chimeric antibodies, humanizedantibodies and a Fab expression library

The terms “binding fragment of an antibody” or “antibody fragment” referto a fragment of a full-length antibody which results e.g. from deletionof N-terminal or C-terminal amino acids of the full-length antibody andwhich maintains the capacity to bind the cognate antigen with about thesame specificity and/or K_(D) as the full-length antibody.

Assays to determine binding specificity and K_(D) of an antibody arewell known and routinely practiced in the art. For a comprehensivediscussion of such assays, see Harlow et al. (Eds.), Antibodies: ALaboratory Manual; Cold Spring Harbor Laboratory; Cold Spring Harbor, NY(1988), Chapter 6.

One approach to measure specificity is by testing an antibody forbinding to its cognate antigen and other non-related antigens usingWestern Blotting or ELISA. Another approach to measure affinity ofbinding of an antibody to its antigen is by surface plasmon resonance(SPR) using e.g. the Biacore™ assay platform and software (GEHealthcare).

Size of latex particles: For commercially available particles, theparticle size is indicated by the manufacturer. For the purpose of thisapplication, particle size is determined using nanoparticle trackinganalysis or the NanoSight®-principle, for example using the NanoSight®NS300 instrument (Malvern Panalytical) which utilizes the properties ofboth light scattering and Brownian motion in order to obtain the sizedistribution and concentration measurement of particles in liquidsuspension. In brief, a laser beam is passed through a sample chamber,containing the particles of interest, suspended in solution. Theparticles in the path of this beam scatter light in such a manner thatthey can be visualized via 20× magnification microscope onto which acamera is mounted. The camera operates at 30 frames per second (fps),capturing a video file of the particles moving under Brownian motion.The software tracks many particles individually and using theStokes-Einstein equation calculates their hydrodynamic diameters. Themeasurement is controlled via Standard Operating Procedures. Moreinformation is available at the manufacturer's website (incorporated byreference).

Blood tests, involving laboratory examination of a sample of blood toobtain information about its physical and chemical properties, todayform the foundation of medical diagnosis. Hundreds of haematologicaltests and procedures have been developed over the years. While the testswere originally performed manually, one by one, the field of clinicalchemistry has become highly automated. Today multiple tests can becarried out simultaneously on one single sample of blood using automatedclinical chemistry analysers, instruments frequently referred to asautoanalyzers.

While the presently disclosed assay method can be performed manually,the assay is preferably performed on an autoanalyzer, as this allowshigh throughput, reduces error and minimizes staffing requirements. Aclosed, automated sample handling also minimizes the risk that staff isexposed to possible blood borne pathogens.

When taking a blood sample, blood is usually drawn from a vein in thearm and collected in standardized sterile blood collection tubes.Different tubes are available, depending on which component orcomponents that is to be analysed. A blood collection tube can be empty,or prefilled with a buffer or more frequently, with an anticoagulant.Examples of anticoagulants include sodium citrate, lithium-heparin, andEDTA. Different combinations are also possible, there are for exampletubes containing sodium heparin and EDTA. The Vacutainer® (BD, Becton,Dickinson and Company) is one example of blood collection tubes,available in different sizes and pre-filled with various reagents.

A distinction is made between plasma samples and serum samples. When aplasma sample is desired, blood is collected in a tube containing ananticoagulant, the tube is turned upside down about 5 to 10 timesdirectly after filling, in order to ensure mixing. The tubes are thenstored in an upright position in room temperature to avoid haemolysis.EDTA and lithium heparin containing tubes can generally be stored up to4 hours in room temperature. Alternatively, the tubes must bedeep-frozen and stored for later analysis.

For obtaining a serum sample, blood is collected in an empty sample tubeor a tube containing a coagulation activator. After filling, the tubesare turned upside down about 5 to 10 times directly after filling, inorder to ensure mixing. The tubes are then stored in an upright positionin room temperature to avoid haemolysis. In most cases, serum tubes canbe stored in room temperature and without centrifuging for up to 4hours. Upon arrival in the laboratory, the tubes are centrifuged, forexample centrifuged for 10 minutes at 2000× g. Following centrifugation,a layer of blood cells can be seen at the bottom of the tube, and thefluid above is called serum. The serum should be separated and storedcold without any additives until transport or analysis.

A first aspect of the present description concerns a method fordetermining the concentration of N-terminal pro-brain natriureticpeptide (NT-proBNP) in a sample, said method comprising the followingsteps:

-   -   contacting the sample with at least one antibody which        specifically binds to NT-proBNP within the amino acid sequence        of SEQ ID NO. 2; and/or    -   contacting the sample with at least one antibody which        specifically binds to NT-proBNP within the amino acid sequence        of SEQ ID NO. 3;        -   wherein said at least one antibody is immobilized to a            particle; and    -   determining a change in reflectance, scattering or transmittance        of the sample wherein said change is indicative of the        concentration of NT-proBNP in the sample.

According to a preferred embodiment of said first aspect, said at leastone antibody specifically binds to NT-proBNP within the amino acidsequence of SEQ ID NO. 2.

The present inventors were surprised to see that an analyte such asNT-proBNP, which is present at very low concentrations, could bereliably detected in an immunoparticle-based assay using polyclonalantibodies raised against a peptide within the amino acid sequence ofSEQ ID NO. 2; and/or a peptide within the amino acid sequence of SEQ IDNO. 3. This is very advantageous, because—when realized in the form of aturbidimetric or nephelometric assay, the determination of theconcentration of NT-proBNP can be performed on any automated clinicalanalyser, and thus leads to higher throughput, shorter response times,and savings in labour and cost. Another advantage is that the use of ashort peptide for raising the antibodies, and in the calibrator, reducesthe cost of manufacturing the immunoparticles and the reagents, aspeptide synthesis is rather costly.

According to an embodiment of said first aspect, the at least oneantibody comprises a polyclonal antibody and/or a mixture of monoclonalantibodies. The use of a polyclonal antibody has an advantage inparticle assisted assays, as a polyclonal antibody is capable of bindingto several epitopes, resulting in higher overall antibody affinityagainst the antigen due to the recognition of multiple epitopes. Thisability to detect multiple epitopes also results in more robustdetection, and a greater sensitivity for detecting antigens that arepresent in small quantities in the sample. Polyclonal antibodies arealso less sensitive to antigen changes such as slight denaturation,polymorphism, or in the case of NT-proBNP, heterogeneity ofglycosylation, than monoclonal antibodies. Polyclonal antibodies alsooffer advantages in terms of short production time and lower cost.

According to an embodiment of the above aspect and embodiments thereof,the polyclonal antibody and/or the mixture of monoclonal antibodiesbinds with substantially same affinity to both glycosylated andnon-glycosylated forms of NT-proBNP or fragments thereof.

Currently the glycosylation sites of NT-proBNP are held to be aminoacids 36, 37, 44, 48, 53 and 58 of SEQ ID NO. 1. Also T71 can beglycosylated, but this is located close to the cleavage site and theconvertase-dependent cleavage of proBNP into NT-proBNP and BNP can onlyoccur if T71 is not glycosylated. Consequently, the majority of theunprocessed proBNP found in circulation has an O-glycan on T71 whereasthe same amino acid in NT-proBNP is not glycosylated (Semenov et al.,Processing of proBNP is suppressed by O-glycosylation on the regionclose to the cleavage site, Clin Chem, 2009, 55(3): 489-498).

Preferably the polyclonal antibody and/or the mixture of monoclonalantibodies specifically recognizes more than one epitope of NT-proBNP.The expression “more than one epitope” includes that different(glycosylated or non-glycosylated) epitopes are recognized, and that aplurality of linear or structural epitopes within NT-proBNP arerecognized.

The inventors postulate that an assay which is less sensitive toglycosylation of the antigen, or even independent thereof, will resultin a more accurate quantification result. In other words, the hereindisclosed assay reflects the true concentration of native NT-proBNP in asample. There are indications that inter-individual variations exist inthe glycosylation patterns (Saenger, A. K. et al., Specificity of B-TypeNatriuretic Peptide Assays: Cross-Reactivity with Different BNP,NT-proBNP, and proBNP Peptides, Clinical Chemistry, 63:1 (2017)351-358), and the inventors postulate that the herein described assayand its components will become the basis for a new standard forNT-proBNP assays. An important advantage of the herein described assayis that it doesn't require a pre-treatment such as de-glycosylation ofthe sample, and thus simplifies handling, increases throughput andremoves a possible source of error, otherwise present when additionalsample handling steps are involved.

It presents an additional challenge to measure antigen molecules inpatient samples when the antigen molecules are present in very lowconcentrations. Antigen NT-proBNP concentrations in patient plasmasamples at low concentrations, below 200 ng/l and even below 100 ng/l,have previously not been possible to quantify by turbidimetricmeasurements, partly because of low turbidimetric signal strengthsrelated to the low peptide concentrations, and partly because theglycosylation blockage of detecting antibodies in large parts of theNT-proBNP molecules.

According to an embodiment, the kit includes a mixture ofimmunoparticles wherein said mixture comprises both immunoparticlescoated with polyclonal antibodies or a mixture of monoclonal antibodieswhich binds to NT-proBNP within the amino acid sequence of SEQ ID NO. 2,and immunoparticles coated with polyclonal antibodies or a mixture ofmonoclonal antibodies which binds to NT-proBNP within the amino acidsequence of SEQ ID NO. 3.

In an especially advantageous embodiment of the method disclosed herein,immunoparticles coated with antibodies having a high affinity for thepeptide of SEQ ID NO. 2 or a fragment thereof are combined withimmunoparticles coated with antibodies having a high affinity for thepeptide of SEQ ID NO. 3 or a fragment thereof. As a result, a large partof the non-glycosylated sequences of NT-proBNP molecules could beutilized for generating the turbidimetric signal by bindingimmunoparticles with the antibodies against both peptides of SEQ ID NOs2 and 3 or fragments thereof, using either polyclonal antibodies or amixture of monoclonal antibodies, having specific affinity to thepeptides of SEQ ID NOs 2 and 3, and/or fragments thereof.

The two peptides identified and chosen by the inventors, i.e the regionsrepresented by SEQ ID NO:s 2 and 3 are both outside the glycosylatedregion of native NT-proBNP. Several other regions and putative epitopes,partially overlapping or falling within these sequences have beendisclosed and used for raising monoclonal antibodies. It is presentlyheld by the inventors that the specific peptides, represented by SEQ IDNOs 2 and 3 or parts thereof, can be used for raising polyclonalantibodies, and optionally also for purifying the antibodies. Asdisclosed in the context of Example 1, the peptides can be used forraising antibodies, in lieu of the native or recombinant full-lengthNT-proBNP, and/or for purifying the resulting mixture of antibodies.

Preferably the immunization is performed with a non-glycosylatedrecombinant antigen, such as one or both the peptides represented by SEQID NO. 2 and SEQ ID NO 3, as well as fragments thereof, followed bypurification using peptides representing epitopes similar or identicalon both glycosylated and non-glycosylated antigens. Using these peptidesor peptide fragments as immunogens has an advantage in that thesynthesis of a shorter peptide is less costly than the synthesis of thefull-length molecule. Different approaches to raising antibodies andpurifying the same are shown in Table 2.

According to an embodiment of the above aspect and embodiments thereof,the polyclonal antibody and/or the mixture of monoclonal antibodiesbinds/bind specifically to NT-proBNP with a K_(D) to recombinantNT-proBNP of less than M, such as less than 5.0E-09 M, preferably lessthan 2.0E-09 M.

According to another embodiment, the polyclonal antibody and/or themixture of monoclonal antibodies binds/bind specifically to the peptideof SEQ ID NO. 2 or a fragment thereof with a K_(D) of less than 5.0E-09M, preferably less than 2.0E-09 M.

According to yet another embodiment, the polyclonal antibody and/or themixture of monoclonal antibodies binds/bind specifically to the peptideof SEQ ID NO. 3 or a fragment thereof with a K_(D) of less than 10.0E-09M, preferably less than 5.0E-09 M, more preferably less than 2.0E-09 M.

According to another embodiment of the first aspect, freely combinablewith the above embodiments, said at least one antibody is a polyclonalantibody or a mixture of monoclonal antibodies, preferably a polyclonalantibody, binding to at least one epitope chosen from the epitopesconsisting of the amino acids of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID.NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9 and/or SEQ. ID.NO. 10.

The method according to claim 1, wherein the particle has an average(mean) size in the range of 150-300 nm, preferably about 150-240, morepreferably 180-220 nm in uncoated form. Particle size can be determinedusing a NanoSight® NS300 instrument (Malvern Panalytical Ltd., Malvern,United Kingdom) according to the manufacturer's instructions.

When a concentration of NT-proBNP is measured nephelometrically,particles with a size in the upper end of the interval are used, andwhen the concentration of NT-proBNP is measured turbidimetrically,smaller particles can be used. The inventors have tested particles withan average size of 50 nm, 100 nm, 140 nm, 150 nm, 200 nm, 240 nm, and290 nm and shown that the method works well with particles starting froman average size of 150 nm. A preferred size interval is 160 nm-240 nm,and in particular 180 nm-220 nm. When the particle size exceeds 200 nm,the spontaneous sedimentation of the particles however has to beaddressed, for example by adjusting the density of the storage bufferfor the immunoparticles. The present inventors have found that thestability can be improved, and the sedimentation of particles can beprevented or at least significantly reduced by consciously adjusting thedensity of the buffer in which the particles are suspended, and thatthis can be done without compromising the activity of theimmunoreagents, the optical properties of the resulting solution, or theperformance of the assay.

A suitable density adjusting agent can be chosen from heavy water(deuterium oxide) and non-ionic water-soluble natural and/or syntheticpolymers. The density adjusting agent is preferably chosen from5-[acetyl-[3-[˜{N}-acetyl-3,5-bis(2,3-dihydroxypropylcarbamoyl)-2,4,6-triiodoanilino]-2-hydroxypropyl]amino]-1-˜{N},3-˜{N}-bis(2,3-dihydroxypropyl)-2,4,6-triiodobenzene-1,3-dicarboxamide(iodixanol, available e.g. as OptiPrep™) and polysucrose (e.g. Ficoll®)and most preferably iodixanol.

Based on their experiments, the inventors concluded that particles inthe range of about 180-220 nm in uncoated form are most suitable whenthe method is performed as a turbidimetric method.

The inventors have surprisingly shown that the coating ratio hasrelevance for the sensitivity of the method, and thus according to anembodiment of said first aspect, said at least one antibody is coatedonto the particle at a coating ratio of 6%, (0.6:10)-60% (6:10),preferably 6% (0.6:10)-30% (3:10), most preferably 10% (1:10)-20%(2:10).

According to an embodiment of the above, the coating ratio is chosenthus, that said at least one antibody is coated onto the particlesforming at least one layer, or layers, having a thickness of at leastabout 10 nm, preferably about 20 nm, and most preferably about 30 nm ormore.

Higher coating ratios have also been tested, in particular coatingratios >100%, i.e. where the amount of antibodies exceed the amount oflatex particles at the onset of the coating reaction. Without wishing tobe bound by theory, the present inventors believe that a high coatingratio may result in “over-loaded” or “super-loaded” particles, which mayincrease the binding speed of antigens to the particles. The results ofthe particle size measurements described in Example 4 indicate that athick layer or multiple layers of antibodies can indeed be coated ontothe particles.

According to an embodiment of the first aspect, and any embodimentsthereof, a change in reflectance, scattering or transmittance, isdetermined at a wavelength in the range from 350 to 700 nm, preferablyat about 450-550 nm, for example at a wavelength of 546 or 548 nm. Aperson skilled in the art will realize that the wavelength may beadjusted depending on other parameters of the assay, mainly particlesize.

According to another embodiment of the first aspect, freely combinablewith any of the other embodiments, the method includes a step ofcalibration using a calibrator comprising a known concentration of asubstance chosen from the peptide of SEQ ID NO. 2, a fragment thereof,the peptide of SEQ ID NO. 3, or a fragment thereof, for example a knownconcentration of a substance chosen from the peptide of SEQ. ID. NO. 4,SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID.NO. 9 and SEQ. ID. NO. 10, or combinations thereof. In the alternative,the calibrator is chosen from native or recombinant full-lengthNT-proBNP of SEQ ID NO. 1.

It is advantageous to use a short peptide in the calibrator, as it willbe faster and cheaper to synthesize a shorter recombinant peptide thanto isolate the antigen from natural samples, for example patientsamples.

According to another embodiment, the change in reflectance, scatteringor transmittance of the sample is a change in transmittance, and themethod is performed as a turbidimetric measurement of the concentrationof NT-proBNP.

According to another embodiment, the change in reflectance, scatteringor transmittance of the sample is a change in reflectance or scattering,and the method is performed as a nephelometric measurement of theconcentration of NT-proBNP.

A second aspect of the present disclosure relates to an immunoparticlecoated with at least one antibody chosen from a polyclonal antibody ormixture of monoclonal antibodies which specifically binds to NT-proBNPwithin the amino acid sequence of SEQ ID NO. 2; or which specificallybinds NT-proBNP within the amino acid sequence of SEQ ID NO. 3.

According to a preferred embodiment of said second aspect, said at leastone antibody specifically binds to NT-proBNP within the amino acidsequence of SEQ ID NO. 2.

According to an embodiment of said second aspect, the polyclonalantibody and/or the mixture of monoclonal antibodies binds withsubstantially same affinity to both glycosylated and non-glycosylatedforms of NT-proBNP.

According to a further embodiment, the polyclonal antibody and/or themixture of monoclonal antibodies binds specifically to NT-proBNP withinthe amino acid sequence of SEQ ID NO. 2 or the amino acid sequence ofSEQ ID NO. 3 and exhibits a K_(D) to NT-proBNP of less than 5.0E-09,preferably less than 2.0E-09 M. In a preferred embodiment, the affinityto NT-proBNP is determined by measuring the K_(D) to recombinantNT-proBNP.

According to a further embodiment, the at least one antibody is apolyclonal antibody or a mixture of monoclonal antibodies, preferably apolyclonal antibody, binding to at least one epitope chosen from theepitopes consisting of the amino acids of SEQ. ID. NO. 4, SEQ. ID. NO 5,SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9, and/orSEQ. ID. NO. 10.

According to an embodiment of said second aspect, freely combinable withall other embodiments, said immunoparticle has a core consisting of aparticle having an average size in the range of 150-300 nm, preferablyabout 150-240 nm, more preferably 160-240 nm, most preferably 180-220 nmin uncoated form. In one embodiment, the particle is a latex particle.

A third aspect relates to a kit for particle enhanced opticaldetermination of a concentration of NT-proBNP in a sample, said kitcomprising:

-   -   at least one antibody which specifically recognizes and binds to        NT-proBNP within the amino acid sequence of SEQ ID NO. 2 or SEQ        ID NO. 3, wherein said at least one antibody is immobilized to a        particle;    -   a calibrator, and optionally,    -   instructions for using the kit to determine the concentration of        NT-proBNP in the sample.

Preferably the particle is an immunoparticle according to the secondaspect or any embodiment thereof.

According to an embodiment of said third aspect freely combinable withthe above, the calibrator comprises a known amount of the peptide of SEQID NO. 2, a fragment thereof, the peptide of SEQ ID NO. 3, or a fragmentthereof, or a combination thereof. In the alternative, the calibrator ischosen from native or recombinant full-length NT-proBNP of SEQ ID NO. 1.

The kit according to the above, further comprising a storage buffer inwhich the immunoparticles are suspended, wherein the storage buffercomprises a density gradient medium and the density of said storagebuffer is adjusted to substantially the same specific weight as thespecific weight of the immunoparticles. A suitable density adjustingagent can be chosen from heavy water (deuterium oxide) and non-ionicwater-soluble natural and/or synthetic polymers. The density adjustingagent is preferably chosen from5-[acetyl-[3-[˜{N}-acetyl-3,5-bis(2,3-dihydroxypropylcarbamoyl)-2,4,6-triiodoanilino]-2-hydroxypropyl]amino]-1-˜{N},3-˜{N}-bis(2,3-dihydroxypropyl)-2,4,6-triiodobenzene-1,3-dicarboxamide(iodixanol, available e.g. as OptiPrep™) and polysucrose (e.g. Ficoll®)and most preferably iodixanol.

According to an embodiment, the kit further comprises an assay buffer,said assay buffer comprising PEG in a concentration of 0.05-0.5,preferably 0.05-0.4% weight/volume.

According to another embodiment, freely combinable with otherembodiments of this third aspect, the assay buffer comprises a blockingagent, for example 0.001-2% of a suitable protein such as albumin ofdifferent species (bovine, rabbit, chicken). According to one embodimentthe blocking agent is IgG in a concentration of about 0.1%.

According to yet another embodiment, freely combinable with otherembodiments of this third aspect, said particle enhanced optical methodis chosen from nephelometry and turbidimetry.

A fourth aspect of the present disclosure relates to a method foridentifying a subject with heart failure or having an elevated risk forheart failure by determining the concentration of NT-proBNP in a bodilyfluid sample of said subject, said method comprising:

-   -   providing an immunoparticle according to the second aspect or        any embodiments thereof;    -   reacting the sample with said immunoparticle;    -   detecting a change in reflectance or transmittance of the        sample, wherein the change in reflectance, scattering or        transmittance of the sample is indicative for the amount of        NT-proBNP; and    -   determining if said concentration of NT-proBNP in said sample is        elevated above a selected control cut-off concentration;        wherein said elevated concentration of NT-proBNP is an        indication that the subject has heart failure or an elevated        risk for heart failure.

According to an embodiment of said fourth aspect, the sample is a samplechosen from blood plasma and blood serum and the change in reflectance,scattering or transmittance is a change in transmittance and the methodis a turbidimetric method.

According to an embodiment of the above, the sample is a sample chosenfrom blood plasma and blood serum and the change in reflectance,scattering or transmittance is a change in reflectance or scattering andthe method is a nephelometric method.

It is a significant advantage that the method can be performed asturbidimetric or nephelometric method, as such methods can be automated,and performed on existing clinical analysers, available in practicallyall clinical chemistry laboratory globally.

A fifth aspect relates to a polyclonal antibody or a mixture ofmonoclonal antibodies, binding specifically to NT-proBNP and whichexhibits a K_(D) to recombinant NT-proBNP of less than 5.0E-09,preferably less than 2.0E-09 M.

According to an embodiment of the fifth aspect, said polyclonal antibodyor mixture of monoclonal antibodies is capable of specificallyrecognizing and binding to both glycosylated and non-glycosylated formsof recombinant NT-proBNP with substantially same affinity.

Preferably said polyclonal antibody or mixture of monoclonal antibodiesis further capable of specifically recognizing and binding to NT-proBNPwithin the amino acid sequence of SEQ ID NO. 2 and/or the amino acidsequence of SEQ ID NO. 3.

According to an embodiment of the fifth aspect, the polyclonal antibodyor mixture of monoclonal antibodies are capable of binding to at leastone epitope chosen from the epitopes consisting of the amino acids ofSEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO. 7, SEQ. ID.NO. 8, SEQ. ID. NO. 9 and/or SEQ. ID. NO. 10.

A sixth aspect relates to the use in vitro of a polyclonal antibodywhich specifically binds to NT-proBNP and which exhibits a K_(D) torecombinant NT-proBNP of less than 5.0E-09 M in diagnosis, monitoring,stratifying or predicting mortality rate in patients with heart failureor identifying subjects at risk of developing heart failure.

According to an embodiment thereof, said polyclonal antibody binds to asynthetic peptide chosen from the peptide comprising or consisting ofthe amino acids of SEQ ID NO. 2, and the peptide comprising orconsisting of the amino acid residues of SEQ ID NO. 3.

According to an embodiment of the sixth aspect, said polyclonal antibodyor mixture of monoclonal antibodies, preferably a polyclonal antibody,binds to at least one epitope chosen from the epitopes consisting of theamino acids of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID.NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9 and/or SEQ. ID. NO. 10.

The results achieved by the inventors indicate that NT-proBNP can bemeasured quantitatively using turbidimetric methods. In addition, oralternatively, the choice of antibodies, the excess loading ofantibodies to the particles, and/or the choice of epitopes, can make itpossible to measure a higher proportion of the total NT-proBNP in thesample, e.g. even 5 times higher concentrations compared to currentcommercial assays which may only measure non-glycosylated NT-proBNP.

Specific advantages have been presented in association with selectedfeatures and other advantages will be apparent to a person skilled inthe art upon study of the following examples which illustrate theinventions.

EXAMPLES Example 1. Production of Antibodies

Antibodies were produced in either a mammalian host (rabbit or goat), orin an avian host (hens) against different immunogens and purified asshown in Table 2 below.

TABLE 2 Overview of antibody production Host Immunogen Purified againstMammalian NT-proBNP NT-proBNP (recombinant, entire molecule, (Rabbit(recombinant, non-glycosylated, HyTest) or entire Peptide 1 (SEQ. ID.NO. 2) goat) molecule) Peptide 2 (SEQ. ID. NO. 3) proBNP (recombinant,glycosylated, HyTest) Peptide 1 + NT-proBNP (recombinant, entiremolecule, KLH non-glycosylated, HyTest) Peptide 1 (SEQ. ID. NO. 2)proBNP (recombinant, glycosylated, HyTest) Peptide 2 + NT-proBNP(recombinant, entire molecule, KLH non-glycosylated, HyTest) Peptide 2(SEQ. ID. NO. 3) proBNP (recombinant, glycosylated, HyTest) AvianNT-proBNP NT-proBNP (recombinant, entire molecule, (hen) (recombinant,non-glycosylated, HyTest) entire Peptide 1 (SEQ. ID. NO. 2) molecule)Peptide 2 (SEQ. ID. NO. 3) proBNP (recombinant, glycosylated, HyTest)Peptide 1 NT-proBNP (recombinant, entire molecule, non-glycosylated,HyTest) Peptide 1 (SEQ. ID. NO. 2) proBNP (recombinant, glycosylated,HyTest) Peptide 2 NT-proBNP (recombinant, entire molecule,non-glycosylated, HyTest) Peptide 2 (SEQ. ID. NO. 3) proBNP(recombinant, glycosylated, HyTest)

Antibodies against NT-proBNP and variants thereof (for examplerecombinant or native, truncated or non-truncated forms, glycosylatedand non-glycosylated forms, and combinations thereof) can be producedaccording to methods known to a person skilled in the art, either inmammal hosts, e.g. goats or rabbits, or preferably in hens, in whichcase the antibodies can be isolated from the egg yolk. Two peptidesdesignated “Peptide 1” and “Peptide 2” were synthetized (GenscriptBiotech (Netherlands) B.V., Leiden, The Netherlands), delivered as awhite lyophilized powder with >90% HPLC purity, and used as immunogens,in addition to the recombinant, full-length NT-proBNP in glycosylatedand non-glycosylated form:

HPLGSPGSAS DLETSGLQEQ RNHLQGKLSE LQVEQTSLEPLQESPRPTGV WKSREVATEG IRGHRKMVLY TLRAPR (Full-length NT-proBNP, SEQ ID NO. 1) SAS DLETSGLQEQ RNHLQGKLSE LQV (SEQ ID NO. 2, Peptide 1, amino acid residues 8-33 of SEQ ID NO. 1)IRGHRKMVLY TLRA  (SEQ ID NO. 3, Peptide 2, amino acid residues 61-74 of SEQ ID NO. 1)

When raising antibodies against a shorter peptide, such as Peptide 1 orPeptide 2, these are preferably conjugated to a suitable carrier toincrease immunogenicity. One example is keyhole limpet hemocyanin (KLH)used in the immunization of the mammalian hosts (rabbits). Immunizationand antibody purification protocols for raising polyclonal rabbitantibodies are well-known to a person skilled in the art, and availablefor example from CBG, Max Planck Institute of Molecular Cell Biology andGenetics. General procedures are also disclosed in Antibodies: ALaboratory Manual, Edward A. Greenfield (Ed.), Cold Spring HarborLaboratory, Cold Spring Harbor, N.Y., 2012.

Avian immunoglobulin fractions such as those used in this assay can bepurified from eggs from immunized hens, immunized with native orrecombinant NT-proBNP or variants thereof, as indicated in Table 2. Theantigen and suitable adjuvant, e.g. complete Freund's adjuvant, isinjected subcutaneously and/or intramuscularly into breast tissue of thehen. Immunizations are repeated at regular intervals, for example on day10, 20 and 30. Antibodies are usually detected in the eggs by day 30.

Eggs are collected, the yolk separated from the white, and theimmunoglobulin fraction is isolated from the egg yolk. As a first step,the lipids and lipoproteins are removed, using a suitable method knownto persons skilled in the art, for example precipitation using PEG ordextran sulphate, solubilisation using organic solvents, orultrafiltration. The resulting substantially lipid-free solution canthen be concentrated and purified to obtain the desired immunoglobulinfraction containing polyclonal antibodies against the antigen.

The immunisation of rabbits was performed by GenScript Biotech Corp.,Piscataway, NJ, USA, and the immunization of hens by NorwegianAntibodies AS, NABAS, As, Norway. All antibodies were purified by GeticaAB, Gothenburg, Sweden.

Goat IgG was produced by InnovaGen AB, Lund, Sweden, immunizing goatswith a peptide according to SEQ ID NO. 2 and SEQ ID NO. 3, following thesame methods, mutatis mutandis. Methods for the concentration and/orpurification of immunoglobulins are also well known to a person skilledin the art and include precipitation steps using for example PEG orsodium sulphate, or other well-known methods such as ultrafiltration orliquid chromatography. For more guidance, see e.g. Antibodies: ALaboratory Manual, Edward A. Greenfield (Ed.), Cold Spring HarborLaboratory, Cold Spring Harbor, N.Y., 2012.

Polyclonal antibodies against the peptides of SEQ ID NO 2 and SEQ ID NO.3 also achieved high affinity (See Example 2 below) and highspecificity, avoiding binding to glycosylated regions, which allows themeasurement of the true NT-proBNP concentration with high sensitivityalso in a turbidimetric or nephelometric setting.

Example 2. Biacore™ Affinity Analysis

As part of the quality assurance in the production of antibodies for theexperiments disclosed herein, affinity analysis was routinely performedfor each batch. The affinity of an antibody for its antigen isdetermined by surface plasmon resonance (SPR) in a method often referredto as the Biacore™ method. SPR occurs under conditions of total internalreflection of plane-polarized light at an interface between two media ofdifferent refractive index, coated with an electrically conducting film.When a sample binds to the surface, the refractive index increases, andthe resonance angle changes. The angle is monitored continuously as theSPR signal, and a plot of the signal against time, called a sensorgram,shows the changes in refractive index as a sample binds to anddissociates from the surface.

The analysis was performed using a Biacore™ X100 system, using the GESensor Chip CM5 (all from GE Healthcare). The chip has a gold surfacewith a dextran matrix having carboxyl groups on it. For determining theaffinity of an antibody to an antigen, the surface of a chip isactivated, and an antigen is covalently attached to the surface. Thisprocess is irreversible, so the chip can be re-used several timeswithout the need for extra antigen.

For determining the kinetic constants, the target antibodies were firstdialysed against PBS and then diluted in HBS to 30 nanomolar (nM). Adilution series was prepared (30 nM-10 nM-3.3 nM-1.1 nM and 0.37 nM) foreach target antibody. The concentration was confirmed byspectrophotometric analysis at 280 nm.

The antibodies were analysed on the Biacore™ X100 system, using theBiacore™ X100 Control Software, which when executed on the systeminjects the antibody dilution series in reverse (starting with the mostdiluted) sequentially and as a final step the surface is re-generatedwith glycine (pH 1.5). The kinetic constants are extracted from thesensorgram using the Biacore™ X100 Evaluation Software based on a 1:1kinetic model. FIG. 3 show the sensorgram from a Biacore™ affinityanalysis for IgG antibody from rabbit antiserum, purified againstnon-glycosylated recombinant NT-proBNP, and tested against a Biacore™chip with the full-length antigen. The equilibrium dissociation constantwas determined to KD=1.35E-09 M.

Similarly, FIG. 4 shows the Biacore™ affinity results for IgY antibodypurified against Peptide 1 (SEQ ID NO. 2) and tested against a Biacore™chip with the full-length antigen. The equilibrium dissociation constantwas determined to KD=2.11E-09 M. Typical results of the Biacore™analysis are summarized in Table 3 below:

TABLE 3 Dissociation constants Biacore ™ data (dissociation constant,K_(D), Host Immunogen Purified against molar (M)) Mammalian RecombinantNT- Recombinant 1.35E−09 (rabbit) proBNP, entire NT-proBNP, moleculeentire molecule, non-glycosylated Peptide 1 8.67E−10 Peptide 2 1.26E−09Avian (hen) Recombinant NT- Recombinant Average 5E−09 proBNP, entireNT-proBNP, (several different molecule entire molecule, lots tested)non-glycosylated Peptide 1 1.98E−09 Peptide 2 1.72E−07 proBNP 5.24E−09(recombinant, glycosylated)

The results showed that the immunisation protocol and purificationconsistently produced antibodies binding specifically to NT-proBNP andexhibiting a K_(D) to recombinant NT-proBNP of less than 2.0E-09 M.

The affinity constants were plotted against the span response and thesensitivity of the assay when run on a clinical chemistry analyser(Mindray® BS-400). As seen in FIGS. 5 and 6 , there is a clearcorrelation between span, sensitivity and the dissociation constants.

IgG antibody against the peptide of SEQ ID NO 2 from goat antiserum,purified against the same peptide, was tested against a Biacore™ chipwith the peptide of of SEQ ID NO 2. The equilibrium dissociationconstant was determined to KD=2.64E-09 M.

IgG antibody against the peptide of SEQ ID NO 3 from goat antiserum,purified against the same peptide, was tested against a Biacore™ chipwith the peptide of of SEQ ID NO 3. The equilibrium dissociationconstant was determined to KD=9.72E-09 M.

Similarly, IgG antibody against the peptide of SEQ ID NO 3 from rabbitantiserum was tested against a Biacore™ chip with the peptide of of SEQID NO 3. The equilibrium dissociation constant was determined toKD=6.54E-09 and 5.23E-09 M.

Example 3. Epitope Mapping

Epitope mapping was performed by Pepscan Presto BV, Lelystad, TheNetherlands, using libraries of linear and looped peptide arrayssynthesised onto a solid support covered with a hydrogel. The mappingtechnique was first published in 1984 (Geysen et al., Use of peptidesynthesis to probe viral antigens for epitopes to a resolution of asingle amino acid, PNAS, Jul. 1, 1984 81 (13) 3998-4002, incorporated byreference). Chicken and rabbit antibodies were provided by Gentian AS,and the mapping focused on a 26 residues long segment of the NT-proBNPsequence, positions 8-33, sequence SASDLETSGLQEQRNHLQGKLSELQV (identicalto SEQ ID NO. 2).

Synthesis of peptides: To reconstruct epitopes of the target molecule alibrary of peptide-based peptide mimics was synthesized using Fmoc-basedsolid-phase peptide synthesis. An amino functionalized polypropylenesupport was obtained by grafting with a proprietary hydrophilic polymerformulation, followed by reaction witht-butyloxycarbonyl-hexamethylenediamine (BocHMDA) usingdicyclohexylcarbodiimide (DCC) with N-hydroxy-benzotriazole (HOBt) andsubsequent cleavage of the Boc-groups using trifluoroacetic acid (TFA).Standard Fmoc-peptide synthesis was used to synthesize peptides on theamino-functionalized solid support by custom modified JANUS liquidhandling stations (Timmerman et al., (2007) Functional reconstructionand synthetic mimicry of a conformational epitope using CLIPS™technology, J. Mol. Recognit. 20:283-299; and Langedijk et al. (2011)Helical peptide arrays for lead identification and interaction sitemapping, Analytical Biochemistry 417:149-155, incorporated byreference).

In a first set, linear peptides of a length ranging from 5-24 derivedfrom the target sequence/SEQ ID NO. 2) were synthetized with an offsetof one residue. In a second set, constrained peptides of length 7-27were synthetized starting from Linear peptides of length 5-25 residuesderived from the target sequence of NT Pro BNP with an offset of oneresidue were placed between two Cys residues. Cys residues were joinedby mP2 CLIPS in order to create a loop mimic.

The binding of antibody to each of the synthesized peptides was testedin an ELISA. The peptide arrays were incubated with primary antibodysolution overnight at 4° C. After washing, the peptide arrays wereincubated with a 1/1000 dilution of an appropriate antibody peroxidaseconjugate for one hour at ° C. After washing, the peroxidase substrate2,2′-azino-di-3-ethylbenzthiazoline sulfonate (ABTS) and 20 μl/ml 3%H₂O₂ were added. After one hour, the color development was measured. Thecolor development was quantified with a charged coupled device(CCD)-camera and an image processing system.

The main epitope candidates identified in this study are presented inthe table below:

TABLE 4 Epitope candidates based on   epitope mapping Amino acid SEQUENCE ID.  sequence NO. SASDL SEQ. ID. NO. 4 DLETSGLQEQRNHLSEQ. ID. NO. 5 NHLQGKL SEQ. ID. NO. 6 SASDLETS SEQ. ID. NO. 7 ETSGLQSEQ. ID. NO. 8 GKLSELQ SEQ. ID. NO. 9 GLQEQR SEQ. ID. NO. 10

Example 4. Coating of Latex Particles

Latex particles (Ikerlat Polymers S.L., Spain) of different sizes werecoated with antibodies produced as in Example 1. Particles of differentsizes were investigated. The following particle sizes (reported by thesupplier) were investigated: 50, 100, 140, 150, 200 and 290 nm. Based ontheir experiments, the inventors concluded that particles in the rangeof about 180-220 nm in uncoated form are most suitable when the methodis performed as a turbidimetric method. But also particles in the rangeof 150 nm-180 nm perform well in the method. For nephelometric methods,also encompassed by the present description and claims, immunoparticlesup to a size of 320 nm were found to be very usable, while the bestresults were obtained using particles having a size of 270-300 nm.

The following general coating procedure was followed: An aliquot ofantibodies (e.g. 10 mg IgG) was dialysed into a boric acid and NaClbuffer, pH 9.5. When the particles were not already chloromethylated, anactivating step was included before the particles were mixed with IgG atvarious coating ratios. The mixtures where then incubated, for exampleat 37° C. for 3 days. A blocking buffer comprising 1 mg/ml rabbit serumalbumin in boric buffer was added, and the mixture incubated overnight.A first dilution buffer comprising 10 mM boric acid, 15 mM NaCl,glycine, Tween®, ProClin™, and rabbit serum albumin, pH 9.5 was added,and the mixture incubated overnight at 37° C.

The particles with conjugated antibodies were then washed twice with adialysis buffer comprising 5 mM boric acid, 7.5 mM NaCl, Tween®,ProClin™, and rabbit serum albumin, pH 8.8, and finally washed twicewith TRIS buffer, pH 8.8.

In one experiment, 150 nm latex particles were coated with goat IgGantibodies raised against the peptide of SEQ ID NO. 2 as follows:

14 mg goat anti human NT-proBNP (Getica AB Sweden) antibodies weredialyzed in a Slide-A-Lyzer® 10000 MWCO in 7.5 mM NaCl, 5 mM boric acid,pH. 9.33 resulting in about 14 mg antibodies in 56 ml 7.5 mM NaCl, 5 mMboric acid, pH 9.3 at the end of the dialysis.

A 4% particle suspension (0.625 ml=0.625*40=25 mg) of 150 nm diameterlatex particles (Thermo Fisher Scientific, Cat. No. C 29369, packaginglot number 1555355) was washed by dialysis in purified water and mixedwith the above 14 mg goat IgG, and reacted under stirring during 24hours at 44° C. The resulting mixture was centrifugated and thesupernatant removed by suction.

To this 10 ml of a solution of 5 mM boric acid, 7.5 mM NaCl, 1.5 mMglycine 0.25 g/l Tween® 20, 0.25 g ProClin™ per l with 1 mg/ml goatalbumin was added. The mixture was stirred for 10 hours at 44° C.

The resulting mixture was centrifuged at 8000 rpm for 60 minutes. Thesupernatant was removed by suction, and the mixture resuspended in 25 mlbuffer (7.5 mM NaCl, 5 mM boric acid, 1.5 mM glycine, 0.25 g/l Tween®20, 0.25 g ProClin™ per with 1 mg/ml goat albumin). This step wasrepeated 3 times.

The resulting particles were suspended in 10 ml storage buffercomprising 7 mM boric acid, 7.5 mM NaCl, 1.5 mM glycine, 0.25 g/l Tween®20, 0.25 g ProClin™ per l, and 1 mg/ml goat albumin.

Different amounts of IgG were loaded onto latex particles, investigatingthe effects of different coating ratios, e.g. 6%, 15%, 60%, 90%, 120%and 150%. The term “coating ratio” here refers to the amount of antibodyin relation to the amount of latex particles at the start of the coatingreaction. For example, 0.5 mg IgG to 1 mg latex particles represents acoating ratio of 50%, 1 mg IgG to 1 mg particles represents a ratio of100% and so on.

Example 5. Particle Size Analysis

An immunoparticle lot was made with 200 nm latex particles and a 10%coating ratio following the general procedure disclosed in Example 4 andsubjected to particle size measurement. FIG. 7 shows the sizedistribution of immunoparticles determined using a NanoSight® NS300instrument (Malvern Panalytical Ltd., Malvern, UK). The results confirmthat a substantially homogenous lot was achieved.

The size of uncoated and coated particles (latex particles, IkerlatAJ20CH2CI-L3, 199±2 nm, Ikerlat Polymers S.L., Spain) was analysed usinga NanoSight® NS300 instrument (Malvern Panalytical Ltd., Malvern, UnitedKingdom) according to the manufacturer's instructions.

The results are summarized in Table 5 below and illustrated in FIG. 8 .

TABLE 5 Particle size measurements Plain Plain Ratio Particle latex,latex, Coated Coated, Difference Difference IgG/ Lot Mean mode mean Modemean mode Latex L18a 187 183 193 195 6 12   6% L18b 187 183 232 198 4410  15% L18c 187 183 264 216 77 32  60% L18d 187 183 228 210 41 27  90%L18e 187 183 216 207 29 24 120% L18f 187 183 226 205 38 22 150%

The size of an antibody can be estimated to about 10 nm in length. Thus,the results of the measurements done on the particles L18a-f, indicatethat overloading the particles with antibodies results in more than onelayer of antibodies on the particles. An even monolayer wouldtheoretically increase the particle diameter by about 20 nm, and adouble layer by about 40 nm. The diameter increase of 77 nm which wasachieved at the 60% ratio (IgG/latex) indicates that up to four layersof antibodies are present on the particle. There however appears to be adrop-off in coating efficiency at about 50-60% ratio of antibodies tolatex. Without wishing to be bound by theory, the inventors believe thatthe antibodies do not form even layers, but instead accumulate on theparticle, forming a porous network of antibodies.

Again, without wishing to be bound by theory, the inventors believe thatfor small analytes, the analyte can penetrate into this network ofantibodies. Thus, a particle overloaded with antibodies as disclosed inthe present description and examples, is likely to have a significantlylarger effective surface than a conventionally produced particle.

Preliminary experiments indicated that the sensitivity and responseincrease with increasing antibody loading. Based on the results of themeasurements done on lots L18a through L18f, it is concluded thatoverloading the particles with antibodies resulted in the creation ofmore than one layer of antibodies on the particles. Based on thediameter increase, it is contemplated that overloading the particlesresults in structures as illustrated in FIG. 9 .

Example 6. Development and Validation of Assay Components

Immunoassay systems require calibration protocols that are normally moresophisticated than many analytical techniques in routine clinical use.Calibrators used in such assays may differ significantly from theanalyte in clinical specimens. Differences in the properties ofcalibrators, or reference materials, from those of clinical specimensmay include the species origin of the calibrator for an analyte;integrity of the molecular species; matrix of the calibration solution,and addition of preservative agents.

Brain natriuretic peptide (BNP) and the N-terminal fragment of the BNPprecursor (NT-proBNP) are widely used biomarkers for heart failure (HF).Since the discovery of BNP in 1988, much effort has been allocated tothe precise detection of BNP and NT-proBNP levels for reliable HFdiagnostics. As a result, measurements of these biomarkers are globallyaccepted and used in clinical practice for the diagnosis of acute andchronic HF, risk stratification, and monitoring response to therapy.Several immunoassays specific for BNP and NT-proBNP are currentlycommercially available. Recent comparative studies however indicate thatthere are marked differences between different BNP and NT-proBNP assaysand platforms, and the results of measurements are not sufficientlycomparable.

This lack of equivalence between the assays complicates theinterpretation of the results and as a result, the cut-off points usedfor diagnostic decisions are method dependent. Presently, there is noagreement on what kind of BNP or NT-proBNP standard should be used forcalibration, and certified reference material as well as referencemeasurement procedures are lacking.

The multiple circulating BNP fragments, along with proBNP and NT-proBNP,collectively form the B-type natriuretic peptide family. Immunoassaysfor BNP utilize a variety of antibodies, both monoclonal and polyclonal,and diverse calibrator materials. Therefore, there are substantialdifferences in patient specimens between BNP methods, even for assaysthat use identical antibody configurations on different analyticalplatforms.

NT-proBNP assays are generally considered harmonized because all utilizecalibrators from the same manufacturer (Roche Diagnostics) which arethen configured to other manufacturers' immunoassay platforms. However,there are still analytical differences between NT-proBNP assays fromdifferent manufacturers that can be attributed partially to differentspecimen types and platforms, indicative of a larger need for globalstandardization of NP-proBNP assays.

Previous studies have demonstrated that proBNP and NT-proBNP areglycosylated to varying degrees that can interfere with commercialimmunoassays used to quantify NPs in clinical practice (e.g. Saenger etal., 2017). The extent to which these immunoassays exhibitcross-reactivity to B-type natriuretic peptides as well as with theirglycosylated and non-glycosylated forms is an important question toelucidate, as this may affect the clinical performance of the assayswith implications for patient care. Furthermore, there are currently noformal efforts to standardize or maintain harmonization of BNP orNT-proBNP assays, primarily owing to the known differences in antibodiesused and lack of a primary reference standard material.

Calibrator

The results generated by the immunoassay method disclosed herein is ofcourse very much influenced by the values assigned to the calibratormaterial used in the turbidimetric immunoassay. The values assigned tocalibrator material is well described in the prior art, a good examplebeing the protocols developed by Dr. Blirup-Jensen (Blirup-Jensen, S. etal., Protein Standardization IV: Value Transfer Procedure for theAssignment of Serum Protein Values from a Reference Preparation to aTarget Material, Clin Chem Lab Med 2001; 39(11): 1110-1122).

The values for the calibrator materials used in the turbidimetricimmunoassays disclosed herein was regulated to suit the target for thevalue transfer procedure of the assay according to Blirup-Jensen et al.

The calibrator material can be constituted by serum or plasma materialsfrom human or animal serum or plasma materials. The calibrator materialcan further comprise a salt solution, like solutions of PEG-6000 (orother PEG materials) and preservative agents. The composition of thecalibrator can be adjusted until the assay method generates resultscorresponding to values obtained for reference samples (e.g. assayresults obtained with commercial assay reagents).

In one experiment, plasma samples with concentration of NT-proBNP(determined by the commercial Roche method) of 4040 ng/L and 5480 ng/Lwere pooled for further dilution with a low plasma sample (65 ng/L-Rochemethod). The samples 4040 ng/L and 5480 ng/L were analysedturbidimetrically on a Mindray® clinical analyser and returnedabsorbance values between 2000 and 3000.

The final concentration of the pooled plasma sample was calculated to4330 ng/L, and from this, 5 calibrators were made by diluting the4330-sample with the low sample into the following range: 150 ng/L, 300ng/L, 800 ng/L, 1500 ng/L, 3200 ng/L. The low sample (65 ng/L) was usedas the 0 calibrator.

In another experiment, a stock solution of the peptide of SEQ ID NO. 2was prepared from dry peptide, supplied by Genscript, The Netherlandsand dissolved in 50 mM TRIS, 150 mM NaCl, 0.05% BSA, 0.01% Tween® 20,0.1% ProClin™, pH=7.8, resulting in a stock solution of 900 μg peptideper ml.

A calibrator for calibrating the assay was made by dilution of saidstock solution in plasma from a blood donor testing below 20 ng/lobtained from an external blood bank.

Full-length recombinant NT-proBNP can also be used as calibrator.Calibrators were prepared in TRIS-HCl buffer using recombinant NT-proBNP(>95% purity, Catalogue No. 8NT2, HyTest Ltd., Turku, Finland) toestimated concentrations 94, 188, 376, 750 and 1500 ng/L respectively.Calibration curves were recorded using Lot15B, where antibodies wereraised as disclosed in Example 1 and coated onto particles as disclosedin Example 3. The following assay buffer and application settings wereused. R1: 12.5 mM MOPS, 600 mM NaCl, % PEG, pH 7.2, and theR1-R2-Sample-Wavelength-Reading cycle was: 150-20-15-380-43/79. Thecalibrator set was stored at 4° C. for 2 months before another curve wasrecorded, using the same measuring conditions. FIG. 10 shows theexcellent stability of the calibrator.

In industrial assay production it is not practical to use materialstotally or even partly based on patient serum and plasma. In the methodsdisclosed herein, antigenic material such as the synthetic peptides ofSEQ ID NO. 2 and SEQ ID NO. 3 or fragments thereof is/are used togenerate a turbidimetric or nephelometric signal based on reactionsbetween the antigen and immunoparticles carrying antibodies binding toSEQ ID NO. 2 and SEQ ID NO. 3 or fragments thereof. Also the full-lengthNT-proBNP molecule can be used as a calibrator.

The composition of the reagents is/are then adjusted, for example asdescribed by Blirup-Jensen et al. so that the NT-proBNP values from theturbidimetric immunoassay is calibrated to the target material, and theassay results in values according to the calibration which is the aim ofthe assay. The assay buffer can for example be adjusted to give aresponse corresponding to the response obtained from a referencepreparation. The assay buffer can for example be adjusted untilcorresponding to the response from a Roche reference assay if the Rochecalibration is the target calibration.

Assay Buffer

An appropriate assay buffer was prepared by mixing soluble substanceslike polyethylenglycol and other soluble polymers and salts and pHbufferingsubstances, and the composition adjusted until theturbidimetric signal of the assay results in a calibration of the assaycorresponded to the target calibration, such as the Roche NT-proBNPassay calibration. The assay buffer composition can also be adjusted sothat the results correspond to a calibration using a reference methodmeasuring the total concentration of NT-proBNP, comprising bothglycosylated and non-glycosylated forms of NT-proBNP, or to other targetcalibrations corresponding to reference and/or standardized calibrationsfor methods for the determination of NT-proBNP in samples of bodyliquids.

The salinity of the assay buffer was varied by adding different amountsof NaCl to the buffer, for example 150, 210, 225, 300 and 450 mM NaCl,added to 12.5 mM MOPS, 0.4% PEG, 0.1% IgG, 0.1% ProClin™, and pH 7.2

The immunoparticles (Lot 46) were supplied in the same storage buffer(R2) in both runs: 1 mg/mL immunoparticles suspended in Tris buffer.First, a 2 mg/mL solution of the immunoparticles was sonicated for 30sec before dilution to 1 mg/mL with Tris buffer. The assay settings werethe same in both runs performed on a Mindray® clinical analyser: R1 150μL|R2 25 μL|sample 5 μL. The calibrator range was0-150-300-800-1500-3200 ng/L.

The results show that the assay could be fine-tuned by adjusting theionic strength of the assay buffer, and the response values could beadjusted to correspond with analysis of the corresponding samples usingthe Roche Assay reagents at the Academic University Hospital in Uppsala,Sweden. The results are shown in FIG. 11 .

Example 7. Analysis of Patient Samples

28 frozen plasma samples with NT-proBNP concentrations ranging from0-4430 ng/L (assigned with Roche Elecsys®) were obtained.

Calibrators were prepared by first dissolving Peptide 1 (GenscriptBiotech (Netherlands) B.V., Leiden, The Netherlands) in Tris-HCl buffer.The dissolved Peptide 1 was then added to NT-proBNP free plasma (plasmafrom a blood donor testing below 20 ng/l obtained from an external bloodbank) to estimated concentrations 0, 130, 500, 1000, and 4000 ng/L,respectively, adjusted to the Roche method.

Antibodies were raised as disclosed in Example 1 and coated ontoparticles as disclosed in Example 3.

A particle-enhanced turbidimetric immunoassay (PETIA) was set up. Anassay buffer comprising 12.5 mM MOPS, 210 mM NaCl, 0.4% PEG, 0.1% IgG,0.1% ProClin™, pH 7.2 was used. The assay can be operated manually butis preferably run on an automated clinical chemistry instrument. Aperson skilled in the art can optimize the parameter settings for theassay on any suitable automated clinical chemistry instrument.

The assay was run on a clinical chemistry analyser (Mindray® BS-400,Shenzhen Mindray® Bio-Medical Electronics Co., Ltd., China) with thefollowing settings shown in Table 6 where theR1-R2-Sample-Wavelength-Reading cycle was: 150-25-5-546-43/79.

TABLE 6 PETIA parameters in Mindray ® BS-400 Parameter Setting Samplevolume [μL] 5 R1: Assay buffer volume [μL] 150 R2: Reagent volume [μL]25 Wavelength [nm] 546 Reading time cycle 43-79

The assays according to the present invention are based on antibodiesdirected towards the peptides of SEQ ID NO. 2 and SEQ ID NO 3, regionsof the NT-proBNP molecules that are not covered by glycosylation. Asdescribed in Example 6, the calibration of the turbidimetric NT-proBNPimmunoassay of the invention can be adjusted to obtain good correlation.

Calibration curves are shown in FIGS. 12 and 13 , for peptide 1 (SEQ IDNO. 2) and peptide 2 (SEQ ID NO. 3) respectively. FIG. 12 shows acalibration curve for lot 46 (200 nm particles coated with antibodiesraised against the peptide of SEQ ID NO. 2 (Peptide 1)), using the samepeptide (SEQ ID. 2) as calibrator antigen. FIG. 13 shows a calibrationcurve for lot 49 (200 nm particles coated with antibodies raised towardsthe peptide of SEQ ID NO 3 (Peptide 2)), using native antigen ascalibrator antigen. It is seen that antibodies are capable of reactingwith varying concentrations of antigen in a linear fashion.

FIG. 14 is a graph comparing the results (ng/l) of 28 turbidimetricmeasurements of NT-proBNP using immunoparticles coated with antibodiesagainst peptide 1 (SEQ ID NO. 2) to the assigned values of the samepatient samples measured using the Roche Elecsys® assay.

FIG. 15 shows the same data as FIG. 14 , here only including sampleswith a measured NT-proBNP concentration lower than 1000 ng/L.

The inventors have shown that it is feasible to measure NT-proBNP in aparticle-enhanced turbidimetric immunoassay. In the Roche Elecsys®sandwich immunoassay, the immunoassay antibodies are however influencedby glycosylation moieties, as described in Røsjø et al. Full correlationbetween the assays of the present innovation and the Roche Elecsys®NT-proBNP immunoassay cannot be expected, and due to said interference,differences between the two assays must be expected.

The turbidimetric assay was also run on another instrument, theArchitect c4000 clinical analyzer, with necessary adjustments of thewavelength, reading interval etc. Calibrator values were adjusted toalign with the Roche method.

TABLE 7 PETIA parameters in Abbot Architect c4000 Parameter SettingSample volume [μL] 5 R1: Assay buffer volume [μL] 150 R2: Reagent volume[μL] 25 Wavelength [nm] 548 Reading time cycle 18-18 (blank) 32-33(reading)

The results confirmed that the method is applicable to being run ondifferent platforms. FIG. 16 shows that sample measurements recorded onthe Mindray® and Architect instruments correlate very well.

Example 8. Mixture of Immunoparticles

An assay using a mixture of immunoparticles was also tested in the sameset-up as disclosed in Example 7, with the difference that a mix of twodifferent batches of immunoparticles was used to make the reagent bufferR2: 200 nm latex nanoparticles coated with antibodies against thepeptide of SEQ ID NO. 2 (Lot 46), and 200 nm latex nanoparticles coatedwith antibodies against the peptide of SEQ ID NO. 3 (Lot 49) were mixedin the ratio 1:0.6.

The assay buffer R1 had the following composition: 12.5 mM MOPS, 210 mMNaCl, 0.4% PEG, 0.1% IgG, 0.1% ProClin™, pH 7.2. The assay was run on aMindray® BS-400 instrument using exactly the same settings as shown inTable 6.

FIG. 17 shows that immunoparticles coated with antibodies raised againstPeptide 1 (SEQ ID NO. 2) can be mixed with immunoparticles coated withantibodies raised against Peptide 2 (SEQ ID NO. 3) and produce afunctioning assay. The response correlates linearly with increasingconcentrations of NT-proBNP, a set of calibrators produced from nativeantigen by mixing patient samples as described in Example 6.

In FIG. 18 , the results (ng/l) of 14 turbidimetric measurements ofNT-proBNP are compared to the assigned values of the same patientsamples measured using the Roche Elecsys® assay. The results show thatimmunoparticles coated with antibodies raised against different peptidescan be successfully mixed, resulting in a functional assay. The resultsfurther indicate that the mixture of immunoparticles is binding tomultiple non-glycosylated regions of the NT-proBNP molecules, therebyresulting in a stronger turbidimetric signal and the detection also oflower concentrations.

Without further elaboration, it is believed that a person skilled in theart can, using the present description, including the examples, utilizethe present invention to its fullest extent. Also, although theinvention has been described herein with regard to its preferredembodiments, which constitute the best mode which is set forth in theclaims appended hereto.

Thus, while various aspects and embodiments have been disclosed herein,other aspects and embodiments will be apparent to those skilled in theart. The various aspects and embodiments disclosed herein are forpurposes of illustration and are not intended to be limiting, with thetrue scope and spirit being indicated by the following claims.

REFERENCES

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1. A method for determining the concentration of N-terminal pro-brainnatriuretic peptide (NT-proBNP) in a sample, said method comprising thefollowing steps: (i) contacting the sample with a mixture of monoclonalantibodies comprising at least a first monoclonal antibody whichspecifically binds to NT-proBNP within the amino acid sequence of SEQ.ID. NO. 2 and at least a second monoclonal antibody which specificallybinds to NT-proBNP within the amino acid sequence of SEQ. ID. NO. 3, andconducting a step of calibration using a calibrator comprising a knownconcentration of a substance chosen from a peptide having the amino acidsequence of SEQ. ID. NO. 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID. NO.7, SEQ. ID. NO. 8, SEQ. ID. NO. 9, or SEQ. ID. NO. 10; wherein said atleast one monoclonal antibody is immobilized to a particle, and whereinthe calibrator is non-glycosylated; and (ii) detecting a change inreflectance, scattering or transmittance of the sample wherein saidchange is indicative of the concentration of NT-proBNP in the sample. 2.The method according to claim 1, wherein said at least one firstmonoclonal antibody specifically binds to NT-proBNP within the aminoacid sequence of SEQ. ID. NO 4, SEQ. ID. NO 5, SEQ. ID. NO. 6, SEQ. ID.NO. 7, SEQ. ID. NO. 8, SEQ. ID. NO. 9, or SEQ. ID. NO.
 10. 3. The methodaccording to claim 1, wherein said at least one monoclonal antibodyspecifically binds to NT-proBNP within the amino acid sequence of SEQ.ID. NO 5, SEQ. ID. NO. 7, or SEQ. ID. NO.
 8. 4. The method according toclaim 3, wherein said at least one monoclonal antibody specificallybinds to the amino acids of SEQ. ID. NO.
 8. 5. The method according toclaim 1, wherein said at least one monoclonal antibody is bound to alatex particle having an average size in the range of 150-300 nm.
 6. Themethod of claim 1, further comprising, on the basis of saidconcentration of NT-proBNP in the sample, diagnosing, monitoring, orstratifying patients with heart failure or subjects at risk ofdeveloping heart failure.
 7. The method according to claim 1, whereinsaid at least one monoclonal antibody binds specifically to NT-proBNPwith a K_(D) to NT-proBNP of less than 10.0E-09 M.
 8. The methodaccording to claim 7, wherein said at least one monoclonal antibodybinds specifically to NT-proBNP with a K_(D) to NT-proBNP of less than5.0E-09 M.
 9. The method according to claim 8, wherein said at least onemonoclonal antibody binds specifically to NT-proBNP with a K_(D) toNT-proBNP of less than 2.0E-09 M.
 10. The method according to claim 1,wherein said at least one monoclonal antibody binds specifically to apeptide having the amino acid sequence of SEQ. ID. NO 5, SEQ. ID. NO. 7,or SEQ. ID. NO. 8 with a K_(D) of less than 5.0E-09 M.
 11. The methodaccording to claim 10, wherein said at least one monoclonal antibodybinds specifically to a peptide having the amino acid sequence of SEQ.ID. NO 5, SEQ. ID. NO. 7, or SEQ. ID. NO. 8 with a K_(D) of less than2.0E-09 M.